It has been found, whether due to leaks or spills, that the soil conditions under and around underground fuel storage tanks frequently included fuel or other VOC contaminants. Test borings can be advantageously employed to determine the presence of such contamination, as determined by the concentration of hydrocarbons and other compounds. Such tests can be used to determine total fuel hydrocarbons, as well as concentrations of specific contaminants such as benzene, toluene and xylene. The tests have shown that often the majority of such contaminants reside in the soil above a depth of forty feet. However, soil and leak rate conditions or volume of spill determine the depth of contamination.
The normal procedure when soil contamination is detected is to determine the cause and achieve a remedy. Then the soil must be decontaminated. A common practice for doing so has been to excavate the contaminated soils and dispose of them at a facility approved for acceptance of hazardous waste. That, of course, has high direct costs, and could interrupt normal business operation for several days, possibly even as long as several weeks
Vapor extraction systems for removing VOC contaminants from soil in situ have been developed and proven to be effective. The vapors are collected as extraction gas and removed therefrom by several possible means, including carbon adsorption, incineration and catalytic oxidation. Catalytic combustion may be the preferred process according to some experts but applicability is constrained within certain limits For example, if the concentration of hydrocarbon vapors exceeds 25% of the lower explosive limit (L.E.L.) for gasoline, heat given off during oxidation raises the temperature of the catalytic combuster to a point of destruction. To a certain extent control of the temperature may be obtained by using dilution air ahead of the catalytic combuster, but this method of temperature control is not practical at the high concentrations of VOC vapor expected during the early phase of the vapor extraction process.
Adsorption of VOC vapors on activated carbon has been found not to be a satisfactory solution for the disposal of recovered vapors. In this process vapors are merely concentrated on a solid bed which must be periodically backflushed to rejuvenate the carbon. The problem of dealing with hydrocarbon contaminants then arises all over again.